Generally speaking, an internal power line is may be used as the data transmission path of a gas centralized monitor system in collective residences such as condominiums in order to reduce system cost. Since a variety of electric appliances are connected to the power line, both the impedence and noise level through the power line are randomly variable. As a result, if the receiving level remains stationary and the signal level decreases, the needed data will be missed or the signal itself may not be received due to the presence of noise components.
The present invention relates to a means for providing an optimum signal receiving level under such circumstances. FIG. 1 shows the construction of a centralized monitor system, where the transmission sub units 1 each incorporating a sensor, monitor panel 2 as the main unit, and the power line 3 as the data transmission path, are characteristically provided.
The monitor panel 2 transmits an answer back instruction in order to periodically check a variety of the existing conditions of the sub units. Assuming that the period of such a checking operation is "r", if the monitor unit still cannot receive any data from any of the sub units even after a maximum of 2r hours have past, then it shows that the pre-set signal receiving level is too low. On the other hand, there is a certain noise component caused by a variety of electric appliances, which constantly flows through the power line at a much higher level than the white level. As a result, if the received signal is too sensitive, the monitor panel cannot correctly identify if it is a signal or noise, and then all the signals cannot be received. Actually, the optimum receiving level exists between the ranges of the signal and noise.
In general, since there are a variety of noises that flow through the power line, no data can effectively be transmitted merely by lowering the signal sensitivity after detecting any of the noise components.
Co-pending application Ser. No. 436,931 filed on Oct. 27, 1982, to Ise et al. and entitled "Noise Reduction In Signal Transmission System Over Building Power Distribution Wiring" discloses a transmission system comprising means related to a data transmission method using the power supply line in order to send and receive the carrier to and from the indoor distribution line, synchronous with the power source frequency. This system uses a phase period, for example, on half of the power cycle, as the period corresponding to the period of the power voltage against the binary data "1" and "0". Here, two bits are used to transmit one bit of data by transmitting 10 (or 01) for a data "1" and 01 (or 10) for a data "0", whereas, during the signal reception period, either the counted number of pulses responding to the carrier existing in the corresponding phase period or the difference of the sum of the carrier duration time is read out, and receives only such data which exceeds the pre-set value.
For example, the operation of this system may be shown in FIG. 2. If the noise components (shown by fine tilt lines) and the signal components (shown by coarse tilt lines) coexist and if the detection level is set at position "a", the signal can be correctly detected, although a certain amount of noise may remain. If the detection level is at position "b", it cannot be identified as either a signal or noise, and no signal can be received. If the detection level is at position "c", neither noise nor signal can be detected. The method embodied in the invention related to the co-pending application eliminates the cumulative difference as a noise component if it is within the preset value. However, as is clear from FIG. 2, after the software procedures are completed, none of the combinations other than the basic bits 01 and 10 may be generated in principle. Receipt of such bits as 00 and 11 is therefore caused solely by inadequate sensitivity.
In other words, assuming that the half period of the power source corresponds to the basic bits, the four basic bits shown in the table below respectively have the meaning which are caused by inadequate sensitivity in the signal reception.
______________________________________ Basic bits Meanings ______________________________________ 00 Data level is too low 01 Data is "0" 10 Data is "1" 11 Too much noise exists ______________________________________